I believe the following should print only one log message, not two:

import logging
class Filter(object):
    def filter(self, record):
        return record.name == 'foo'  
logger = logging.getLogger()
formatter = logging.Formatter('%(message)s') 
handler = logging.StreamHandler()
handler.setFormatter(formatter)
logger.addFilter(Filter())
logger.addHandler(handler)
logging.getLogger('foo').warn('foo!')
logging.getLogger('bar').warn('bar!')

If the filter is added to the handler instead of the logger, everything works as expected.

If this is desired behaviour, please consider this a bug against the documentation. In that case I propose to extend the documentation of Logger.addFilter() as follows: "Note that the filter will act only on log messages that are generated by this logger, and not on messages that have been generated by descendant loggers."